Ink jet recording has characteristics such that the noise during recording is small, color printing is possible, high speed printing is possible, printing can be made on ordinary paper to have high quality, and it is accordingly widely used for printing by computers irrespective of personal use or office use. Various systems are available for this ink jet printing, including a method wherein an ink in a slender nozzle provided in a recording head, is discharged by static energy, a method wherein an electric current is conducted to a heating zone in a recording head so that bubbles will be formed by the heat generation thereby to discharge the ink from a nozzle to carry out printing.
As an ink to be used for such ink jet recording, it has been common to employ a water-base ink having a dye dissolved or dispersed in water. Such a water-base ink is used also for writing utensils such as fountain pens or ballpoint pens.
The following items may be mentioned as the properties required for a recording ink to be used for such applications.
(1) Printed or written letters will have no bleeding. PA1 (2) Printed or written letters will not be discolored by light or heat. PA1 (3) Even when left to stand for a long period of time, it causes no clogging of a pen point or a nozzle in a recording head. PA1 (4) Its storage stability is good. PA1 (5) The viscosity of the ink is low.
As mentioned above, as an ink for these applications, it has been common to employ an ink having a dye dissolved or dispersed as a coloring agent in water. However, when a dye is employed, there is a problem that bleeding is likely to be observed in the printed or written letters, or discoloration is likely to occur by light. Accordingly, an attention has recently been drawn to an ink which employs carbon black as a black pigment.
However, if such carbon black is used as a pigment for ink, the dispersibility of carbon black in water is poor, since the surface of carbon black is lipophilic, whereby carbon black is likely to cause clogging of a pen point or a nozzle, or aggregates are likely to form during storage, whereby it becomes no longer practically useful. Accordingly, it is required to add various dispersing agent.
Here, as the dispersing agent, a dispersant containing in its molecule both lipophilic groups having good affinity with carbon black and hydrophilic groups having good affinity with water, specifically mainly a resin dispersant, is employed to improve the dispersibility or the dispersion stability.
However, such a resin dispersant is expected to provide its effects as bonded to the surface of carbon black. Accordingly, the dispersing effect can not be obtained unless it is added in an amount in excess of the amount required for bonding to the surface of carbon black. Therefore, an excess dispersant will remain in the liquid, and when the ink at a pen point or in the nozzle, is dried, it turns to a solid which is hardly re-solubilized and which causes clogging. Further, as the dispersant is added, the viscosity of the ink increases, whereby there will be a problem that constant discharge of ink can not be attained.
To overcome these problems, JP-A-8-3498 discloses an attempt to improve the dispersion stability without using a dispersant, by reacting carbon black with a sodium hypochlorite solution to oxidize the carbon black more than the commercial products and to have the surface subjected to hydrophilic treatment. However, by such a method, many chlorine ions and sodium ions used as the oxidizing agent are present in the liquid. Accordingly, to use such a product as an ink, it is necessary that the reaction product is once filtered and then purified by a reverse osmosis membrane or ultra filtration. Further, the oxidation treatment is carried out at a high temperature of 100.degree. C. for a long period of time of 12 hours, whereby humic acid soluble in water, will form.
Further, as a case where commercial carbon black is further subjected to oxidation treatment, JP-A-7-258578 discloses that using a gas phase low concentration ozone, commercial carbon black is subjected to oxidation treatment for a long period of time.
However, with carbon black subjected to oxidation treatment by such a method, the surface of carbon black is remarkably eroded, probably because the action of the oxidizing agent is strong, whereby the surface area increases, and at the same time, humic acid soluble in water will be formed. Further, it has been found by a study by the present inventors that functional groups of carbon black formed by such treatment have no adequate affinity with water, although the reasons are not known, whereby the dispersion stability is inadequate.
Thus, humic acid and sodium ions will be present in large amounts in a water-base ink containing carbon black subjected to oxidation treatment using, as an oxidizing agent, ozone in a gas phase or sodium hypochlorite as the above-described prior art, and, as disclosed for example in JP-B-7-51687, these impurities will become solid at a pen point or in a nozzle and thus cause clogging.
Further, as an oxidation treatment method, JP-A-50-142626 discloses a method wherein, while an aqueous medium and carbon black are stirred and mixed, ozone is supplied through pores from a lower portion of the stirring tank. However, here, a method of dispersing into a hydrophobic vehicle such as oil by flushing (oil is added to the aqueous mixture of a pigment powder with stirring to transfer the pigment powder to the oil phase), is disclosed, and nothing is disclosed with respect to carbon black excellent in the dispersibility in an aqueous medium, which is useful particularly as a pigment for a water-base ink, particularly for an ink for ink jet. It is disclosed undesirable that ozone is introduced in excess of 2.times.10.sup.-5 gr mol per 1 m.sup.2 of the specific surface area of furnace black. Thus, it appears intended that the degree of oxidation of the resulting carbon black should preferably be low. However, no specific degree of oxidation is disclosed. Thus, heretofore, a method has not been found which is capable of obtaining carbon black excellent in dispersibility in an aqueous medium which is useful as a pigment for a water-base ink, particularly for an ink for ink jet, and there has been a problem when carbon black is used as a pigment for a water-base ink.
It has been desired to solve the above-described problem of the prior art and to provide an ink and a pigment for a water-base ink excellent in dispersion stability and excellent in discharge stability, whereby clogging of a pen point or a nozzle can be prevented.
Further, spherical foreign matters, i.e. micro foreign matters of from 1 to 20 .mu.m made of a solidified product of sprayed raw material oil, present in carbon black, are hardly removable from the carbon black. Therefore, at the time of producing a water-base ink or a solvent-containing ink, such carbon black is put into e.g. a beads mill as it is, followed by dispersion to obtain an ink. As a result, the micro foreign matters are disintegrated to fragments of from a few hundred nm to a few .mu.m, whereby not only the size has been reduced, but the number of particles has increased substantially.
An ink containing such foreign matters had a problem such that the foreign matters tended to precipitate during storage of the ink or in the case of a coating material, they became a source for causing hard spots in the coating film, or in a case where coated by a spin coater, the coating film had streaks. Therefore, after the dispersion, it took time for centrifugal separation or filtration, and even then, it was not easy to completely remove them. Further, it was not practically possible to produce carbon black containing little micro foreign matters.
It has been desired to establish production of a pigment, whereby removal of foreign matters is easy, an ink containing little micro foreign matters can efficiently be prepared, and the dispersion stability is excellent and when made into an ink or a coating material, the degree of blackness is high, and the gloss is good, and which is useful for a coating material having good coating film properties.